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We have located the account using your community details. TP link AC1200 ArcherC50 is a dual band router only and for your NBN Hybrid Fibre Coaxial (HFC) service to work, you will need to use a VDSL2+ modem router.
This advice helped me: https://community.tpg.com.au/t5/Broadband-Internet/Alternate-router-to-use-on-HFC/td-p/863
Essentially you need to go to Network -> IPTV settings. Select Custom mode there and assign VLAN ID 2 to the Internet (Remaining IDs can be 3 and 4). And tick 802.1Q Tag checkbox.
After this it all works well.